1. Field of the Invention
The present invention relates to the field of silicon carbide and methods for growing crystals of silicon carbide and is more particularly related to a method for epitaxially growing silicon carbide layers on silicon carbide substrates.
2. Description of the Prior Art
Silicon carbide (SIC) is a semiconductor material which provides many advantages over other available materials. There are two classes of SiC: .beta.-SiC with a cubic (zinc blend) crystal structure and .alpha.-SiC with hexagonal and rhombohedral crystal structures. There are numerous forms of .alpha.-SiC referred to as polytypes determined by the stacking sequence of double layers of Si and C atoms. Examples of the polytypes of .alpha.-SiC include 6H, 4H, 2H and 15R. 6H SiC is characterized by a 6 layer repeat distance and hexagonal crystal structure. .alpha.-SiC crystals have shown promising results for use in wide band gap semiconductors for high temperature and high power microelectronics applications. However, the growth of .alpha.-SiC bulk and epitaxial materials as well as subsequent device processing is at a relatively immature stage in comparison to silicon (Si) and gallium arsenide (GaAs) semiconductors.
The .alpha.-SiC crystal is shown in FIG. 1. In FIG. 1, crystal 10 is shown having three unique crystallographic axes. They are the c-axis (Miller indices &lt;0001&gt; 12 that is perpendicular to the hexagonal basal plane 14; the a-axis &lt;1120&gt; 16 which is parallel to the basal plane 14 and is directed to one of the six vertices 18 of crystal 10; and the prismatic axis &lt;1100&gt; 20 which is also parallel to the basal plane 14 and is directed to any one of the six sides 22 of crystal 10.
Traditionally, the face perpendicular to the c-axis &lt;0001&gt; 12 has been utilized in crystal growth. The earliest work in (6H) .alpha.-SiC relied on small SiC platelets grown by a sublimation process, known as the Lely process. These platelets, of several millimeters in lateral dimension, were oriented with the face perpendicular to the c-axis 12. More recently, SiC boule growth has also yielded c-axis oriented slices.
Examination of the crystal structure of 6H SiC (one of the .alpha.-SiC polytypes) grown along the c-axis 12 is shown in FIG. 2. FIG. 2 shows that the vertical layered structure 24 terminates on one face by carbon atoms 26 and on the other face by silicon atoms 28. It had been shown previously by numerous investigators that successful epitaxial growth of 6H SiC on c-axis material was a strong function of which face of the crystal was used and, importantly, the exact number of degrees the exposed nominal c-axis surface is off-oriented from the c-axis basal plane 30. This strong orientation dependence is thought to be due to the characteristic pattern of steps and terraces that results from various off-orientations from c-axis 12. FIG. 3 illustrates the steps and terraces resulting from polishing an .alpha.-SiC crystal a few degrees off the c-axis 12.
In U.S. Pat. No. 3,956,032, a method is described for growing SiC layers on SiC substrates utilizing an axis transverse to the c-axis. In one embodiment of that method, sections cut from a 6H SiC platelet have opposing a-faces which are parallel to the c-axis of the platelet. The sections serve as substrates for the growth of SiC layers by attaching the substrates to a body which is then placed in a chamber and the chamber evacuated. Hydrogen is then admitted and the body on which the substrates are mounted is heated to produce a temperature profile such that the subsequent admission of a carbon containing chlorosilane gas or a mixture of a chlorosilane gas in a hydrocarbon gas will cause free silicon to be deposited at one end of the body while SiC crystals grow on the substrates which are in a preferred temperature range. The epitaxial growth of the SiC crystals on the transverse axis is achieved in that region of the susceptor having a temperature profile between 1320.degree. C. and 1390.degree. C.
The epitaxial growth method of U.S. Pat. No. 3,956,032 failed to specify the use of the transverse a-axis and failed to specify the a-axis or prismatic orientation of the transverse material. Moreover, the a-axis epitaxial growth method disclosed in U.S. Pat. No. 3,956032 failed to produce a smooth morphology. That growth method produced epitaxial wafers too small for subsequent device processing and too small even for electrical characterization. Accordingly, there is a need for an improved method for epitaxially growing 6H-SiC on the a-axis.